1. Field of the Inventions
The present inventions relate to an exhaust system for a multi-cylinder engine powering a straddle-type vehicle, and in particular, exhaust systems that are adapted to control the area of exhaust passages in accordance with engine conditions.
2. Description of the Related Art
In four-stroke engines, two-stroke engines and the like, it is known that opening and closing of an exhaust valve introduces exhaust into an exhaust pipe intermittently, where an inertia effect and a pulsation effect of the exhaust are produced. These effects (hereinafter called a “dynamic effect”) change in accordance with engine conditions (mainly engine speed).
Therefore, maximizing the dynamic effect to increase volumetric efficiency of the engine at a certain speed significantly reduces the volumetric efficiency at the other speeds due to the dynamic effect working in reverse. Thus, in the case that the features of an exhaust system (the length, diameter and the like of the exhaust pipe) are made so that a more optimal dynamic effect is produced in the high-speed range, a the significant reduction in torque (drop in torque) in the middle-speed range can arise.
Meanwhile, various proposals have been made to improve engine output in a way such that an exhaust control valve adapted to change the area of the exhaust passage is provided in the exhaust pipe, and the exhaust control valve is closed to reduce the area of the exhaust passage and eliminate the dynamic effect in the middle- and low-speed ranges with low volumetric efficiency.
For example, JP-A-Sho 64-15428 discloses an exhaust control system having two sets of exhaust control valves disposed non-coaxially and generally in parallel with each other at the ends of four exhaust pipes, and an interlock mechanism provided at the ends of shafts of the exhaust control valves (see JP-A-Sho 64-15428). When exhaust pressure is exerted on the exhaust control valves, rotational forces produced by the shafts of the exhaust control valves are cancelled through the interlock mechanism, thereby providing reduced load on a motor or the like. This allows reduction of power produced by the motor or the like for driving the exhaust control valves, thereby effecting its size reduction.
JP-A-2002-138828 discloses an exhaust control system for a motorcycle having four exhaust pipes connected into two and then into one, a connecting pipe connected to the front end of a silencer, and a valve mechanism (exhaust control valve) adapted to change the area of an exhaust passage in the connecting pipe, for controlling exhaust pulsation (see JP-A-2002-138828)